In disk storage devices using partial-response (PR) signaling, a receive filter in the readback apparatus has to shape the output signal of the recording channel into a PR signal before a detection device, e.g. a maximum-likelihood sequence detector can reconstruct the recorded data sequence. However, since the recording density of data on a track, and thus the recording channel characteristic varies with track radius, a fixed filter can only shape the channel output signal into a nominal PR signal at a given track; at other radii, the spectrum of the filter output signal deviates from the nominal PR characteristic leading to a degradation in error-rate performance of the recording system. Such a loss in performance can be avoided if an adaptive equalizer is used besides the fixed receive filter, to compensate for variations in channel characteristics with track radius.
Various equalization circuits and methods for improving the operability and effectiveness of recording or communication systems are known in the art.
In the publication "Improvement of recording density by means of a cosine equalizer" by T. Kameyama et al., IEEE Transactions on Magnetics, Vol.MAG-12, No. 6 (November 1976), pp. 746-748, an equalizer was disclosed which improves the recording density in a peak detection magnetic recording system by pulse slimming. The equalizer consists of a delay line, an amplitude divider and a differential amplifier. However, the equalizer is not adaptively updated during reading data from the disk and thus cannot compensate channel characteristic variations.
An article by D. D. Falconer et al. entitled "Application of fast Kalman estimation to adaptive equalization", published in the IEEE Transactions on Communications, Vol.COM-26, No. 10 (October 1978), pp. 1439-1446, suggested employing, for setting of tap coefficients in an adaptive equalizer in data communication systems, a recursive least squares algorithm. This algorithm leads to rapid initial convergence of the equalizer tap coefficients. However, the algorithm requires a great number of multiplications and thus is not suited for high-speed implementation.
U.S. Pat. No. 4,580,176 entitled "Adaptive equalization circuit for magnetic recording channels utilizing signal timing", and the article "Adaptive symmetrical interference equalization" by R. C. Schneider et al., IBM Technical Disclosure Bulletin Vol. 28, No. 11 (April 1986), pp. 4857-4858, disclose adaptive equalizer circuits for magnetic recording systems. However, they are designed for peak detection systems and thus are not suited for recording systems using partial-response signaling.